The present invention relates to a switching method of transmission lines in a transmission network. Specifically, the present invention relates to a method of transmission line switching, transmission equipment, and network architecture suitable to SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy) network.
Recent years, there are proposed many transmission line switching methods to protect signals against line failure (for example, inadvent disconnection or degradation of line, failure of repeaters) in order to improve the reliability of transmission services.
These methods comprise for example: (1) 1 to N type NPS (Nested Protection Switching) network in which a plurality of working lines and protection lines are installed in a same path, and line switching method thereof, (2) 4-Fiber BLSR (Bidirectional Line Switching Ring) and transmission line switching method thereof, in which a plurality of transmission equipment are connected by the working line and protection line in a ring form. Examples of the former method include ""Nested Protection Switching" T1X1.5/90-132,1992" and Fiber Network Service Survivability, and the examples of the latter include "Bellcore "SONET BLSR Genetic Criteria" GR-1230-CORE,1993".
FIG. 9 shows an example of N-type NPS network. In this figure, 101 through 104 designate transmission equipment. The network example of FIG. 9 is connected as follows: its working line 105 is terminated by the transmission equipment 101 and 102. A working line 106 is terminated, on the other hand, by the transmission equipment 102 and 103. These working lines 105 and 106 are connected by using an Add-Drop Multiplexing equipment in the transmission equipment 102. A working line 107 is terminated by the transmission equipment 102 and 104. And the working line 107 is routed by the transmission equipment 103.
On the other hand, the protection lines 109 through 111 are depicted in FIG. 9 by dotted lines. The protection lines 109 through 111 are all connected to every transmission equipment 101 through 104, being connected by using Add-Drop Multiplexing equipment 114 within respective transmission equipment. Each of transmission equipment has ability of switching between transmission lines, and therein the working lines and protection lines transmit signals bidirectionally.
One to N type NPS network as shown in FIG. 9 may select Add-Drop Multiplexing equipment or repeater for a transmission equipment when required for a working line. This allows the flexibility of the N-type network service to be improved. Also its economical efficiency may be improved, since N working lines share the protection line in this network. Furthermore, this network is predominant in the addition of working lines. For example, when traffics between the transmission equipment 101 and 103 are required to be newly added, it is possible to add working lines of the required capacity easily. Thus, as in the example of working line 108, the line addition may be realized by terminating by the transmission equipment 101 and 103, and by repeating by the transmission equipment 102.
At this point, how to switch when a failure occurs in such an architecture will be described with reference to FIG. 9. The switching method is dependent on following three factors: (1) the position in the transmission equipment at the point where the failure has been occurred; (2) the level of importance of the failure; and (3) the order of the occurrence of failures.
If the first failure of the importance level 3 has been occurred in the working line 105, the working line will be protected by using the protection line 109. In this case the larger the importance level, the faster the protection of the failure will be realized.
If the second failure of the importance level 1 has been occurred in the working line 106, the working line will be protected by using the protection line 110.
If the third failure of the importance level 2 has been occurred in the working line 108, the protection lines 109 and 110 will be required for the protection. However, in this case, the protection lines 109 and 110 are already in use. By comparing the importance level between failures in the protection lines, the importance level of the protection line 109 is three and that of the protection line 110 is one. As the importance level of the protection line 109 is higher than the importance level of failure of the working line 108, the working line 108 will not be protected. In this case the working line 106 will remain protected. Thus the transmission equipment which has detected the failure of the working line 108 should know the working line 108 is denied being protected.
If the fourth failure of the importance level 4 has been occurred in the working line 107, the protection lines 110 and 111 are required for the protection while the protection line 110 are already in use. When referring to the importance level of that protection line, the importance level of the protection line 110 is one, which importance level is lower than the importance level of the working line 107. Thus the protection line 110 will be used for the protection of the working line 107. At this time the fourth failure will be protected, whereas the second and third failures will not.
As described above, the switching decision and switching operation between transmission lines in an NPS network will be done in the transmission equipment which terminates the working line. This means that the transmission equipment should know the information on other transmission lines that the working line requests as a protection line simultaneously. Therefore, whether or not the switching operation is proper should be determined correctly based on the communication of switch control information among respective transmission equipment.
There are proposed such methods as follows, in which the switching operation is to be performed by exchanging the control information in the transmission equipment based on the overhead of SONET/SDH. These include: (1) a method using Automatic Protection Switching bytes (APS bytes) and DCC bytes (e.g., ITU-T(International Telecommunication Union-Telecommunication Standardization Sector), T1X1.5/90-132,1990); and (2) a method using APS bytes and a timer (Tsong-Ho Wu,"Fiber Network Service Survivability" Aretec house,1992 ). In this context the APS bytes indicates the bytes defined in the SONET/SDH for the use of exchanging of control information for transmission line switching on the SONET/SDH. APS bytes are comprised of so-called K1 byte and K2 byte. The use of APS bytes on a Point-to-Point basis may be found in the section 5 of "Bellcore GR-253-CORE," issue 1, December 1993.
Now, SONET, SDH and a network of the present invention conduct digital transmission by using an overhead of transmission frames for digital transmission and by using performing frame phase alignment and stuff control by swapping pointers in the digital transmission, as known well.
The above described first switching method "T1X1.5/90-132" is a method for an appropriate switching of working lines on the basis of comparison of the importance level by transmitting the importance level of the working line using a plurality of DCC bytes.
The above described second switching method "Fiber Network Service Survivability" is a method as follows. The transmission equipment having detected a failure transmits K1 bytes of APS bytes to wait for the response with K2 bytes. The destination node transmits K2 bytes indicating the response when K1 bytes are received, on the assumption that a protection line has been allocated. The source node receiving the K2 bytes indicating this response starts the switching operation. If there exists a request of higher importance level on the route to the destination node, the K1 bytes will not be arrived at the destination node, so that the K2 bytes indicating the response will not be transmitted. Therefore, a timeout will occurr in the transmission equipment which has detected a failure, so that the switching operation will be determined not to be performed.
FIG. 10 illustrates 4-fiber BLSR, one of transmission methods. In FIG. 10, reference numerals 115 through 118 designate transmission equipment. The working lines 119 through 122 and protection lines 122 through 125 are connected in a ring form. Each of these transmission equipment has ability of switching transmission lines such that these transmission equipment transmit bidirectionally signals on the working lines and the protection lines.
Now the basic operation of transmission line switching against failures of lines in this 4 fiber BLSR will be described below. In FIG. 10, when a failure occurs in a working line 122, signals will be protected by using protection line 126. Also, if there are failures on both of the working line 122 and the protection line 126, a detour route is used which is specific to the ring form. That means that in the transmission method of 4-Fiber BLSR signal protection may be achieved by using the protection lines 123 through 125. The protection method in the ring form is characterized in that two routes of clockwise and counter-clockwise directions may be selected. For the transmission method of the 4-Fiber BLSR, there has been proposed a high-speed switching method using solely the APS bytes (Bellcore "GR-1230-CORE", Issue 1, December 1993) in the prior art.